libsnapshot: static link libselinux

libsnapshot_init is linked by microdroid. Adding libselinux as whole_static_libs in `libsnapshot_defaults` will bloat the super image of microdroid.

Instead, statically link libselinux for libsnapshot_init.

On Pixel 7 Pro:

Microdroid super image: 47173836
Baseline (Without this change): 47161548

Bug: 362839899
Test: Build, verify microdroid super image size.
Change-Id: I3c589097173b305c1d39af9edbfb8c5e29f21410
Signed-off-by: Akilesh Kailash <akailash@google.com>